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Discuss selection of material based on application and explain "silicon use as ideal substrate material in MEMS".

Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 10M

2 Answers
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  • Material selection is essential for efficient design of MEMS

  • For MEMS designers, one of the key jobs for achieving the high level of reliability, low unit cost and optimal function performance of micro electromechanical devices is to carefully choose materials from a limited set.

  • Silicon based materials, which have been commonly used in the semiconductor integrated circuit industry, remain primary choices for MEMS

  • These materials form the vast majority of micromachined devices.

  • The restriction to this set of materials ensures compatibility with the process and therefore permits a high degree of integration on a single chip.

  • Though the silicon based material set for MEMS is somewhat restrictive, a wide range of other materials is to be exploited.

  • Fabrication process for glass quartz are mature and well established. For other material such as silicon carbide, new techniques are being explored. Also, materials deriving from the carbon material system, such as diamond and amorphous carbon, have recently emerged as promising candidates to improve MEMS mechanical performance.

  • Besides the silicon based technology, using LIGA technique permits consideration of any material that can be electroplated from solution

  • These facts enrich the inventory of available MEMS materials, Furthermore, NEMS are evolving with new technical applications emerging.

Silicon is an ideal substrate material for MEMS because of the following reasons:

  1. It is mechanically stable and it is feasible to be integrated into electronics on the same substrate.

  2. Electronics for signal transduction such as the por n type piezoresistive can be readily integrated with the si-substrate ideal for transistors.

  3. Silicon is almost an ideal structure material. It has about the same young's modulus as steel [ $\backsim$ 2 x $10^5$ Mpa] but it is as light as aluminium with a density of about 2.3 g/c$m^3$

  4. It has a melting point at 1400 degree celsius, which is about twice higher than that of aluminium. this high melting point makes silicon dimensionally stable even at elevated temperature.

  5. Its thermal expansion coefficient is about 8 times smaller that of steel and is more than 10 times smaller than that of aluminium.

  6. Silicon shows virtually no mechanical hysteresis. It is thus an ideal candidate material for sensors and actuators.

  7. Silicon wafers are extremely flat for coatings and additional thin film layers for wither being integral structural parts or performing precise electromechanical functions.

  8. There is a greater flexibility in design and manufacture with silicon than with other substrate materials. Treatments and fabrication processes for silicon substrates are well established and documented.

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